Internal Threading Questions

[ShagDog]

I am about to try internal threading for the first time. I have several questions. Here is a 1st "box" I made that has a good suction fit on the brass lid to bronze base. It is a small box, about 1.5" diameter and the base is about 1.25 high. See photos of box and interior of base. The next box I am in process of making is 2" diameter aluminum and will be bored to about 1.75 diameter. I want to thread the lid and base on this one.

My lathe has a screw on 1 1/2-8 spindle, so I don't think I can safely thread toward the tailstock. I want to make a very fine thread so as to slow down the feed. I also intend to use the backgear to slow the rpm to about 90. I have Machinery's Handbook (19th), and I cannot find the dimensions for a very fine thread. The finest thread I can find is 1 3/4-20. I am also trying to keep the thread length at a minimum (around 3-4 threads). Obviously, I don't want to crash the lathe into the shoulder of the box.

Not sure how to calculate the dimensions. As stated above, this is my 1st attempt to do internal threading. Tips and suggestions would be appreciated.

 

[Jim F]

You can cut any thread pitch at any dia. you please.

 

[Larry$]

The finest thread I can find is 1 3/4-20.

The diameter doesn't matter! Unless you are matching a die.
I used 40TPI for my lathe center checker.
There are ways of locking a threaded chuck so you can use reverse on the lathe. Lots of advantages to threading away from the chuck.

 

[mmcmdl]

TPI depths are constant for any size thread as Jim stated above and if you're asckeered , thread away from the chuck .

 

[MrWhoopee]

Cut the external thread first, use it as a gage for the internal thread.
My Heavy 10 also has a threaded spindle. I have threaded away from the chuck without issue, I just took it easy. A fine thread in aluminum should pose no problem. Just back the chuck off a little from the shoulder and spin it on by hand. DO NOT try this under power.

 

[ChazzC]

Regardless of what thread you decide upon, I strongly suggest that you work on a couple of practice pieces before attempting on the intricate box, even if you cut the threads early in the process.

 

[ShagDog]

Been boring to get close to size today. Going to go with 1 5/8-24 thread. I have left the box's base on the chuck to finish after I complete the external threaded lid, as suggested by MrWhoopee. I will use another chuck for the lid. Thanks for all the tips.

 

[Smithdoor]

I am about to try internal threading for the first time. I have several questions. Here is a 1st "box" I made that has a good suction fit on the brass lid to bronze base. It is a small box, about 1.5" diameter and the base is about 1.25 high. See photos of box and interior of base. The next box I am in process of making is 2" diameter aluminum and will be bored to about 1.75 diameter. I want to thread the lid and base on this one.

My lathe has a screw on 1 1/2-8 spindle, so I don't think I can safely thread toward the tailstock. I want to make a very fine thread so as to slow down the feed. I also intend to use the backgear to slow the rpm to about 90. I have Machinery's Handbook (19th), and I cannot find the dimensions for a very fine thread. The finest thread I can find is 1 3/4-20. I am also trying to keep the thread length at a minimum (around 3-4 threads). Obviously, I don't want to crash the lathe into the shoulder of the box.

Not sure how to calculate the dimensions. As stated above, this is my 1st attempt to do internal threading. Tips and suggestions would be appreciated.

You make gauge for size of three you need for testing. I did for my sb 9 spindle 1½-8 RH.

If lathe does not have a thread spindle you can use reverse. This great for bind threads. Not this does work if you can not use thread dial.

I use a carriage lock to move the carriage to start position for rh thead use reverse direction.
Now ingauge the haft nut per threading dial repeat as need check with thread gauge till finished.

Dave

 

[SLK001]

Follow Mr. Whoopee's advice. FIRST, cut the external threads to full depth. The part with the external thread will be your TEST GAUGE for completing the internal threads. SECOND, cut a trough on the inside of the part with the internal thread to give your bit a place to begin its cuts. When threading, the back gears are used more than 99% of the time. A beginner should NEVER thread without the back gears engaged. Don't be in a hurry. Take maybe 0.010" cut for the FIRST cut, then no more than 0.005" for all additional cuts. When you are close to your calculated depth, start taking SPRING CUTS, until metal removal is small. TEST the internal thread with your GAUGE after EVERY cut once close to your finished depth.

You basically SNEAK UP on your final depth. Go too fast and you'll overshoot your depth, which means that you'll have to scrap your part and start over.

Be careful and be precise and you'll easily accomplish your goal.

 

[ShagDog]

Thanks for the help. Got it done. Did go with 1 5/8-24 threads. I did thread going toward the chuck. I did use backgear at the slowest speed. The threads on the lid and base have a good fit. Container is aluminum. Approximately 2" OD and 1.25" base height.